build: run docs regen in synth.py (#1059)

diff --git a/synth.py b/synth.py
index 017717d..e68036b 100644
--- a/synth.py
+++ b/synth.py
@@ -27,4 +27,6 @@
 s.move(templated_files / '.kokoro', excludes=['**/docs/*', 'publish-docs.sh'])
 
 # Also move issue templates
-s.move(templated_files / '.github')
\ No newline at end of file
+s.move(templated_files / '.github')
+
+s.shell.run(["nox", "-s", "docs"], hide_output=False)
\ No newline at end of file